Synthesis, characterization and 1D helical chain crystal structure of [Cu(DBA)2(1,10-phen)]n and [Cd(DBA)2(1,10-phen)2] (DBA = benzilic acid)

Article abstract of DOI:10.1016/j.ica.2006.09.027

Two novel complexes [Cu(DBA)₂(1,10-phen)]_n (1) and [Cd(DBA)₂(1,10-phen)₂] (2) [HDBA = benzilic acid: (C₆H₅)₂C(OH)COOH] have been synthesized and characterized by element analysis and fluorescence spectroscopy. The crystal structures of compounds 1, 2 and HDBA (3) were also determined. Complex 1 is a one-dimensional (1D) helical infinite chain, in which [(1,10)-phen]Cu^(II) units were bridged by benzilic acid. Complex 2 is a mononuclear structure, and is self-assembled through π-π stacking interactions to form a 1D helical chain. Compound 3 is self-assembled to form a 1D helical chain through hydrogen bonds interactions. Thermal analyses indicate that complexes 1 and 2 are stable under 200 and 254 °C in solid state, respectively.

Full text of DOI:10.1016/j.ica.2006.09.027

Inorganica Chimica Acta 360 (2007) 1819–1824
www.elsevier.com/locate/ica
Synthesis, characterization and 1D helical chain crystal structure
of [Cu(DBA)2(1,10-phen)]n and
[Cd(DBA)2(1,10-phen)2] (DBA = benzilic acid)
*
Yongcai Qiu, Kunnan Wang, Yan Liu, Hong Deng , Feng Sun, Yuepeng Cai
School of chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
Received 4 August 2006; received in revised form 24 September 2006; accepted 25 September 2006
Available online 12 October 2006
Abstract
Two novel complexes [Cu(DBA)2(1,10-phen)]n (1) and [Cd(DBA)2(1,10-phen)2] (2) [HDBA = benzilic acid: (C6H5)2C(OH)COOH]
have been synthesized and characterized by element analysis and fluorescence spectroscopy. The crystal structures of compounds 1, 2
and HDBA (3) were also determined. Complex 1 is a one-dimensional (1D) helical infinite chain, in which [(1,10)-phen]Cu(II) units were
bridged by benzilic acid. Complex 2 is a mononuclear structure, and is self-assembled through p–p stacking interactions to form a 1D
helical chain. Compound 3 is self-assembled to form a 1D helical chain through hydrogen bonds interactions. Thermal analyses indicate
that complexes 1 and 2 are stable under 200 and 254 ꢀC in solid state, respectively.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Coordination polymer; Helical chain; Crystal structure; Fluorescence
1. Introduction
R-helical chains [Mn(hfac)2] (hfac = hexafluoroacetylacet-
onate) [12]. Hong et al. prepared three helical-chain cop-
per(II) complexes with 4,40-bipy and diphenic acid [13].
Considerable strategies have been developed for the prepa-
ration of helical coordination polymers [14–19]. However,
to the best of our knowledge, no examples of the polymer
structures comprising helical chains from benzilic acid have
been reported.
For metal ions, benzilic (DBA) can provide a variety of
chelating and/or bridging coordination modes displayed by
the carboxylic or hydroxy groups. In particular, two phenyl
rings are not coplanar with each other owing to the steric
hindrance. Once the metal ions combine with DBA
through carboxylic and hydroxy groups from both sides,
distortion at both ends of diphenyl spaced by the central
C atom endows DBA to link the metal ions to helical
chains. Herein, we present the synthesis, X-ray studies
and thermo-stability properties of the compounds 1, 2,
and 3. It is noted that three crystal structures exhibit 1D
helical chain through different actions (covalent bond,
hydrogen bond, and p–p stacking interactions).
In recent years, extensively attention has been paid to
the design and construction of the Metal-Organic Frame-
work (MOF) from building blocks due to their fascinating
structural properties and potential applications in catalysis,
host–guest chemistry, enantioselective separation, electrical
conductivity, and magnetism [1–5]. It has been thought
that many novel properties and potential applications
would emerge from unusual MOF [6,7]. Therefore, the
rational designed synthesis of novel MOF is still a chal-
lenge for chemists.
Molecular structures with helical morphology have
practical implication due to their structure similarities with
DNA [8,9]. van Koten et al. reported a binuclear
[Ag2(N4)2]2+ dication with helical structure [10]. Hannon
et al. described a Schiff base silver(I) double helical com-
plex [11]. Inoue et al. synthesize one dimensional (1D),
*
Corresponding author. Tel.: +86 20 33381880; fax: +86 20 85210763.
E-mail address: dh@scnu.edu.cn (H. Deng).
0020-1693/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.09.027

1820
Y. Qiu et al. / Inorganica Chimica Acta 360 (2007) 1819–1824
2. Experimental
(0.5 mmol) and adjusted with an aqueous solution of
sodium hydroxide (0.1 mol/L) to pH 7.0. The mixture
was continuously stirred for another 30 min and filtered.
Blue lamellar single crystals were obtained from the filtrate
at room temperature for 5d. (75% yield). Anal. Calc. for
C40H28CuN2O6 (%): C, 69.01; H, 4.05; N, 4.02. Found:
C, 69.10; H, 4.09; N, 3.95%. (KBr pellet) (cmꢀ1): 3056,
1651, 1630, 1518, 1489, 1445, 1426, 1335, 1222, 1169,
1052, 1033, 963, 873, 852, 765, 737, 724, 699, 604, 526, 495.
2.1. Materials and general methods
All the materials and reagents were obtained commer-
cially and used without further purification. Elemental
(C, H, N) analyses were performed on a Perkin–Elmer
2400 element analyzer. Thermogravimetric analysis
(TGA) experiments were carried out on a Perkin–Elmer
TGA 7 thermogravimetric analyzer with the heating rate
of 10 ꢀC/min from room temperature to 800 ꢀC under
nitrogen atmosphere. The IR spectra were acquired using
Nicolet Avatar 360 FT-IR spectrophotometer. Proton
NMR spectra were obtained using a Varian Mercury 300
FT-NMR spectrometer. Fluorescence spectra were
recorded with an F-2500 FL Spectrophotometer analyzer.
2.2.3. Synthesis of[Cd(DBA)2(1,10-phen)2] (2)
The same synthetic procedure as for complex 2 was
employed except that CuCl2 Æ 2H2O was replaced by
CdCl2 Æ 6H2O, resulting in prism colorless single-crystals
suitable for X-ray diffraction in 52% yield. Anal. Calc. for
C52H38CdN4O6 (%): C, 67.35; H, 4.13; N, 6.04. Found:
C, 67.40; H, 4.10; N, 6.09%. (KBr pellet) (cmꢀ1): 3048,
1614, 1591, 1515, 1492, 1447, 1426, 1386, 1343, 1200,
1171, 1144, 1049(s), 981, 939, 903, 858, 754, 727, 705,
2.2. Synthesis
1
2.2.1. Synthesis of crystal structure of HDBA
677, 633, 552, 473. H NMR (CD3SOCD3, 300 MHz) d:
Benzilic acids were dissolved in 50% ethanol solution
(15 cm3) with stirring. The colorless single crystals suitable
for X-ray diffraction were obtained by slow evaporation for
7d. Anal. Calc. for C14H12O3 (%): C, 73.67; H, 5.30; O,
21.03. Found: C, 73.71; H, 5.27; O, 19.99%.
9.12–9.15 (m, 4H), 8.94–8.97 (m, 4H), 8.93–8.91 (m, 4H),
8.86–8.89 (m, 4H), 8.27–8.30 (m, 4H), 8.31–8.33 (m, 4H),
8..08 (d, 2H), 8.07 (d, 2H), 8.06 (d, 2H), 8.05 (d, 2H),
7.30 (s, 2H), 7.13–7.17 (m, 4H).
2.3. X-ray crystallography
2.2.2. Synthesis of [Cu(DBA)2(1,10-phen)]n (1)
A mixture of CuCl2 Æ 2H2O (0.5 mmol), 1,10-Phen
(0.5 mmol) and 50% ethanol solution (20 cm3) was stirred
for 30 min at 70 ꢀC, it was then added to benzilic acid
Single crystal X-ray diffraction data collections of 1–3
were performed on a Bruker Apex II CCD diffractometer
operating at 50 kV and 30 mA using Mo Ka radiation
Table 1
Crystallographic data and structure refinement summary for complexes 1–3
Formula
C
14H12O3
C40H29CuN2O6
C52H38CdN4O6
Mr
228.24
697.19
1173.26
Crystal system
Crystal size (mm)
Space group
orthorhombic
0.40 · 0.32 · 0.25
Pna21
monoclinic
0.30 · 0.25 · 0.18
P21/c
monoclinic
0.35 · 0.26 · 0.20
C2/c
˚
a (A)
11.2776(11)
8.6554(9)
24.418(2)
27.756(2)
11.3675(7)
22.7132(15)
26.2200(3)
11.46210(10)
17.4611(2)
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
112.999(5)
123.3220(10)
3
˚
V (A )
383.5(4)
6596.79
4384.95(8)
Z
8
8
4
Dcalc (g/cm3)
1.272
1.404
1.405
h Range (ꢀ)
1.67–27.77
0.089
960
1.96–21.30
0.714
2880
1.86–25.25
0.554
1896
l (mmꢀ1
F(000)
)
Range of h,k,l
Total/independent
reflections
ꢀ14/14, ꢀ11/10, ꢀ31/32
ꢀ25/28, ꢀ11/11, ꢀ23/23
ꢀ31/31, ꢀ13/13, ꢀ20/20
15638 /5464
35731/7341
29749/3977
Parameters
311
883
286
Goodness-of-fit
R1 [I > 2r(I)]a
wR2 (all data)b
0.995
0.0698
0.1621
1.158
0.0812
0.2130
1.085
0.0283
0.0653
P
P
a
R1 ¼ jjF oj ꢀ jF cjj= jF oj.
P
P
2
2 1=2
b
wR2 ¼ f ½wðF 2o ꢀ F c2Þ ꢁ= ðF 2oÞ g
:

Y. Qiu et al. / Inorganica Chimica Acta 360 (2007) 1819–1824
1821
˚
k = 0.71073 A) at 293 K. Data collection and reduction
were performed using the SMART and SAINT software [20].
A multi-scan absorption correction was applied using the
SADABS program [21]. All three structures were solved by
direct methods and refined by full-matrix least squares on
F2 using the SHELXTL program package [22]. Hydrogen
atoms were located from difference Fourier maps and a rid-
ing mode. Crystal parameters and details of the data collec-
tion and refinement are given in Table 1.
3. Results and discussion
The benzilic acid is a multifunctional ligand in that it
has the potential to coordinate in a number of different
ways (Scheme 1) [23–25]. The X-ray crystal structures of
compounds 1, 2, and 3 were determined (Fig. 1). The X-
ray crystal structures of complexes 1 and 2 show that they
HO
O
OH
benzilic acid
a
M
M
O
b
O
O
O
O
OH
M
M
M
c
e
d
O
O
O
O
O
OH
Fig. 1. Benzilic acid structure and the coordination environment of the
metal centers in the complexes.
M
M
M
M
f
are 1D chain polymer and mononuclear structure,
respectively.
O
O
O
O
In 3 (HDBA), there is a cone-shaped geometry around a
carbon center similar to methane (Table 2), and there are
two crossed phenyl rings, with the dihedral angle between
them being 81.6ꢀ. The carboxylate group and hydroxyl
group are non-coplanar. Interestingly, the compound is
O
OH
linked into a 1D helical chain by continuous O–Hꢂ ꢂ ꢂ
O
hydrogen bonds rings (see Fig. 2). The hydrogen bonds dis-
tances are shown in Table 3.
Scheme 1. Structure of benzilic acid and some of its possible coordination
modes.

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Y. Qiu et al. / Inorganica Chimica Acta 360 (2007) 1819–1824
Table 2
Table 3
˚
˚
Selected bond distances (A) and angles (ꢀ)
Hydrogen-bond geometry (A, ꢀ)
HDBA
HDBA
C1–O1
C1–C2
C1–C14
C1–C8
C14–O2
C14–O3
1.411(5)
1.518(6)
1.537(5)
1.540(5)
1.213(2)
1.301(1)
O1–C1–C2
104.3(3)
107.9(3)
111.9(3)
111.3(3)
115.1(3)
124.5(2)
D–Hꢂ ꢂ ꢂA
D–H
Hꢂ ꢂ ꢂA
Dꢂ ꢂ ꢂA
D–Hꢂ ꢂ ꢂA
O1–C1–C14
C2–C1–C14
O1–C1–C8
C2–C1–C8
O2–C14–O3
O1–H1ꢂ ꢂ ꢂO2
0.82
0.82
0.82
0.82
0.82
2.28
2.06
1.95
1.81
1.83
2.638(6)
2.812(5)
2.729(6)
2.632(4)
2.646(4)
107
152
159
175
171
O1–H1ꢂ ꢂ ꢂO20#1
O10–H10ꢂ ꢂ ꢂO2#2
O30–H301ꢂ ꢂ ꢂO10#3
O3–H3Aꢂ ꢂ ꢂO1#3
Complex 1
Cu1–O1
Cu1–O6#1
Cu1–N1
Cu1–N2
Cu1–O4
N1–Cu1–N2
O6#1–Cu1–O4
N2–Cu1–O4
Symmetry code: (#1) 1/2 + x, 3/2 ꢀ y, ꢀ1 + z; (#2) x, y, 1 + z; (#3) ꢀ1/
1.907(6)
1.929(5)
2.010(7)
2.018(7)
2.317(5)
81.7(3)
O1–Cu1–O6#1
O1–Cu1–N1
O6#1–Cu1–N1
O1–Cu1–N2
O6#1–Cu1–N2
O1–Cu1–O4
93.6(2)
89.7(3)
172.7(3)
170.6(3)
94.6(3)
91.3(2)
105.4(2)
2 + x, 3/2 ꢀ y, z.
role in forming the 1D helical chain (Fig. 3). In the chain,
the distance of two adjacent Cu is 7.489 A, and the dis-
tance of three adjacent Cuꢂ ꢂ ꢂCuꢂ ꢂ ꢂCu angles is 98.7ꢀ. The
1D helical chain presents an attractive structure framework
with an unprecedented, very small diameter of approxi-
II
˚
81.1(2)
94.5(3)
N1–Cu1–O4
Complex 2
Cd1–O1
Cd1–N1
2.199(2)
2.373 (2)
2.444(2)
90.1(2)
69.0(2)
79.9(3)
O1#2–Cd1–N1#2
O1–Cd1–N1#2
N1–Cd1–N1#2
O1–Cd1–N2
85.0(2)
114.2(3)
147.8(3)
152.5(3)
86.2(3)
˚
mately 18.5 A.
In complex 2, the CdII center is in a distorted octahedral
geometry by coordinating to two carboxylate oxygen
atoms from two BDA ligands, four nitrogen atoms from
two bidentate ligands chelating two 1,10-phen ligands.
Cd1–N2
O1#2–Cd1–N2
N1–Cd1–N2
N2–Cd1–N2#2
N1#2–Cd1–N2
˚
˚
Symmetry code: (#1) ꢀx, y + 1/2, ꢀz + 1/2; (#2) ꢀx, y, ꢀz + 1/2.
The Cd–O distance is 2.199(2) A, and the two Cd–N dis-
tances are 2.010(7) and 2.018(7) A (Table 2), which are typ-
ical values for Cd–Nphen and Cd–Ocoo coordination
distances. The dihedral angle between the twisting phenyl
rings of BDA ligand is 79.3ꢀ, and the dihedral angle
between the twisting phens is 62.5ꢀ. The combination of
these twists and p–p stacking interactions between phens
results in the formation of a 1D helical chain as shown in
Fig. 4. The centroid to centroid and interplanar distances
of the p–p stacking interactions between phen planes are
In complex 1, each CuII center displays a distorted
square pyramidal geometry with two carboxylate oxygen
donors and a hydroxylate oxygen donor from three benzilic
acid ligands, two nitrogen donors from one bidentate che-
lating 1,10-phen ligand. The distinct different bond lengths
˚
˚
are Cu1–O1 [1.907(6) A] and Cu1–O4 [2.317(5) A], the Cu–
˚
˚
N bond lengths are 2.018(7) A and 2.010(7) A (Table 2).
The crossed phenyl rings of two BDA ligands with the
dihedral angles between them are 78.7ꢀ and 70.5ꢀ, respec-
tively. The complex is linked with Scheme 1a and b coordi-
nation mode and 1,10-phen, mode (b) plays an important
˚
3.799(1) and 3.496(2) A, respectively. As stated by Janiak
[26], the usual p–p interaction is an offset or slipped stack-
ing, i.e., the rings are parallel displaced, and the ring nor-
Fig. 2. Hydrogen-bonds network and the helical chain in complex 1.
Fig. 3. 1D helical chain of CuII complex through covalence bond.

Y. Qiu et al. / Inorganica Chimica Acta 360 (2007) 1819–1824
1823
100
80
60
40
20
0
a
b
0
200
400
600
800
1 000
Temperature/˚C
Fig. 5. (a) TG curve of complex 1 and (b) TG curve of complex 2.
Fig. 4. 1D helical chain of CdIIcomplex through p–p stacking
interactions.
mal and the vector between the ring centroids form an
angle of about 20ꢀ up to the centroid–centroid distances
˚
of 3.8 A.
Complex 1 forms a 1D helical chain polymer with
Scheme 1a and (b) coordination mode, whereas complex
2 with Scheme 1a coordination mode displays another
1D helical chain through p–p stacking interactions between
phens. This may originate from the nature of heterometal-
lic atoms employed. Cu(II) favors a square–planar or
square–pyramidal geometry, whereas Cd(II) tends to adopt
an octahedral geometry owing to the strong Jahn–Teller
effect. Meanwhile, the steric effect of phen and DBA
ligands appears to play a significant role in the construction
of the unique structures of 1 and 2. The large steric hin-
drance of phen and L ligands prevents the formation of
heterometallic layered structures. And it is well known that
metal-involved self-assembly processes can be influenced
by many factors used for crystallization. Besides the funda-
mental and strong metal–ligand coordination driven force
(complex 1) , other relatively weak intramolecular or inter-
molecular interactions such as hydrogen bonding com-
pound 3 and aromatic stacking (complex 2) are also
useful tools in constructing the supramolecular framework
and coordination polymers.
The thermogravimetric analyses (TGA) of complexes
were performed in a N2 atmosphere when it was heated
to 900 ꢀC at a rate of 10 ꢀC minꢀ1. For CuII complexes,
TGA studies indicated that the weight loss in the range
of 200–480 ꢀC is attritubed to the burning DBA group
and 1,10-phen ligand, the final residual weight is 12.01%
(Calc. 11.49%) corresponding to CuO; for Cd(II) com-
plexes, TGA results show that the crystals were unchanged
(0.93 wt%) until 695 ꢀC; whereupon, decomposition
occurred (Fig. 5).
Fig. 6. Liquid solution fluorescent emission spectra of complex 2 at room
temperature.
The fluorescent spectral property of the complexes has
also been studied. Compounds 1, 2, and 3 were dissolved
in the ethanol solution and their fluorescent emission spec-
trum was measured at room temperature, respectively
(Fig. 6). Compound 3 does not show any photolumines-
cence and complex 1 also has no fluorescent emission,
which may be due to the quenching effect of Cu ions.
Whereas complex 2 exhibits two intense fluorescent emis-
sion bands at 368, 383 nm, which may be attributed to
intraligand charge transfer of coordinated phen ligands,
similar to the emission of free phen Æ H2O [27].
4. Summary
In summary, two new complexes [Cu(DBA)3(1,10-
phen)]n and [Cd(DBA)2(1,10-phen)2] have been synthe-
sized. Complex 1 exhibits an attractive rope-like 1D
helical-chain structure framework with unprecedented very

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Y. Qiu et al. / Inorganica Chimica Acta 360 (2007) 1819–1824
˚
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mainly by DBA ligand as an efficient template. In complex
2, (1,10-phen) CdII-based show supramolecular architec-
tures assembled through p–p stacking interactions. This
work shows that the rational selection of benzilic ligand
can yield helical-chain complexes, whereas the different
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Appendix A. Supplementary material
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CCDC 615761, 609309 and 609468 contain the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
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